The present invention relates generally to optical systems for performing a defined function using an optical beam that propagates along an optical path within the optical system, and more particularly, to an optical system and method for folding the optical path of the optical beam into a defined volume of the optical amplifier such that the optical amplifier has at least one of a minimized volume and a reduced operating temperature.
In the past few years, there has been an increased use of laser technology in many technical fields, including not only communications, but also in the manufacturing industry and the medical field. For example, the communications industry has replaced much of its existing electrical wiring with optic cable for the transmission of data and voice. Further, the welding and cutting industry has developed laser technology for cutting and welding, while the medical field has used laser technology to perform surgical procedures and other diagnostic testing. Given its versatility, laser technology is currently being considered for a broad range of uses. For example, laser technology has been discussed as a viable technology for transmitting high quantities of power from one location to another for use as a power source. This technology would not only be useful for remote geographic locations that do not have an existing power grid, but also for space-based applications.
One important aspect of many laser-based technologies is the ability to amplify the optical beam of the laser to a desired power level. For this purpose, optical amplifiers have been developed which amplify an optical beam by impinging the beam on a laser-active material. A second optical beam, referred to as an optical pump beam, energizes the laser-active material and increases the power level of the laser-active material. This power is transferred to the optical beam being amplified as it passes through the laser-active material.
Although conventional optical amplifiers are typically suitable for many laser applications, there are some drawbacks associated with these conventional systems. Specifically, many high gain optical amplifiers are typically designed to have an elongated rod-like geometry. The gain path for an optical beam incident on the optical amplifier is composed of short gain regions followed by long regions having no gain. For applications requiring large laser gains, these conventional rod-shaped amplifiers may become extremely elongated and significantly increase the volume and mass of the laser application. For example, welding and cutting systems that require a relatively large amount of laser power may require an optical amplifier that far exceeds design and size limitations for the welder or cutter. More importantly, in spaced-based applications, where volume and mass are at a premium, the incorporation of a large-scale optical amplifier may not be possible.
In light of this, optical amplifiers have been developed which attempt to minimize the volume and mass of the optical amplifier. One such class of optical amplifiers is typically referred to as a multi-pass optical amplifier. A multi-pass optical amplifier typically controls the path of the optical beam to be amplified such that the optical beam is passed several times through a laser-active material. With each pass, the optical beam is amplified. By using the same laser-active material and directing the optical beam on the same laser-active material, the size of the optical amplifier may be reduced.
For example, U.S. Pat. Nos. 5,546,222 and 5,615,043 both to Plaessmann et al. provide one illustration of a multi-pass laser. Specifically, with reference to FIG. 1, the multi-pass laser 40 disclosed in these patents defines a laser-active material 42 located between two reflectors 44 and 46. A pump beam source 48 is located in close proximity to the multi-pass laser and directs a pump beam 50 via lenses 52 and 54 at the laser-active material. The multi-pass laser further includes an outlet 56 from which the optical beam that is amplified escapes. Further, the multi-pass amplifier of this reference includes a transparent material 58 to direct the optical beam to the reflector 46. In this multi-pass laser system, an optical beam 60 to be amplified enters the optical amplifier and is directed on the laser-active material, where it is amplified. The amplified beam is then reflected back and forth between the reflectors 44 and 46 through the laser-active material until the optical beam exits the optical amplifier. Although this optical amplifier does provide a method for amplifying an optical signal, it does have some limitations.
Specifically, the temperature of laser-active material must be properly regulated to ensure the desired amplification and optical beam quality. Allowing the laser-active material to overheat may not only affect the amplified optical beam, but may also subject the laser-active material to undue stress. For this reason, with reference to FIG. 1A, the conventional multi-pass optical amplifier connects the laser-active material 42 to a thermally conductive housing 61. While this configuration aids in the reduction of heat in the laser-active material, it does have drawbacks.
For example, the heat sink configuration of the conventional multi-pass amplifier illustrated in FIG. 1A removes heat from a direction perpendicular to the path that the optical signal follows through the laser-active material. This, in turn, creates thermal induced gradients perpendicular to the path of the optical beam that may cause distortions in the refractive index of the laser-active material. In this conventional multi-pass optical amplifier, however, the heat sink cannot be placed such that it removes heat in a direction parallel to the path of the optical beam, as it would obstruct the optical beam.
FIG. 2 illustrates a second type of multi-pass optical amplifier disclosed in U.S. Pat. No. 5,553,088 to Brauch et al. This multi-pass optical amplifier 62 includes three active reflectors 64a-c each connected to a separate substrate 66a-c and having individual pump sources 68a-c directed at each active reflector. These active reflectors each include a laser-active layer 70 and a reflective layer 72. To amplify an optical beam, the optical beam is directed at the first active reflector 64a, where it is amplified and reflected to the second active reflector 64b. This is continued for the second and third active reflectors. Advantageously, the substrates 66a-c to which the active reflectors are connected are heat sinks, which remove heat from the active reflectors in a direction essentially parallel to the path of the optical signal impinging on the active reflectors. As such, thermally induced gradients in the index of refraction are reduced.
Although the conventional multi-pass optical amplifier disclosed in the Brauch patent does alleviate some of the problems associated with heat removal, it also has some drawbacks. Specifically, as discussed, it is advantageous to minimize the volume and mass of the optical amplifier. However, the multi-pass optical amplifier illustrated in FIG. 2 only impinges the optical beam once on each active reflector and uses a separate pump beam for each active reflector. The multi-pass optical amplifier of FIG. 2 would require several active reflectors and associated pump beam devices to generate an optical beam with high gain, thereby requiring a multi-pass optical amplifier of an undesirable scale.
With reference to FIG. 3, the Brauch ""088 patent further discloses a device for repeatedly supplying a pump beam to an active reflector. Specifically, this device 74 includes an active reflector 76, a pump beam source 78, and a plurality of reflectors 80-88. Further, the device includes two coupling devices 90 and 92 for directing the optical beam 94 to be amplified to the active reflector 76. The pump beam systematically reflects between the reflectors and the active reflector such that the pump beam impinges on the active reflector eight times.
Although this device illustrates repeatedly providing a pump beam to an active reflector, it does not provide disclosure as to how the device can be used to energize an optical beam by repeatedly providing it to the active reflector, such that both the optical beam and pump beam are repeatedly impinged on the active reflector. Further, the device uses an added reflector 88 located beside the active reflector to properly align the pump beam. This added reflector may not only add additional size to the optical amplifier, it may also reduce the power of the pump beam as it propagates to and from the extra reflector 88 and impinges on the reflector. Further, and importantly, the Brauch ""088 patent does not disclose how the pump and optical beams may be redirected to another active reflector such that the optical beam may be further amplified and the pump beam used to energize another active reflector.
As discussed above, the laser-active material of optical amplifiers produces a relatively large amount of heat that affects the operation of the optical system. In light of this, it is typically advantageous to allow for reduced operational heat. However, in many conventional optical systems, the laser-active elements may be spaced too closely to one another an may overheat.
Further, there are other optical system applications that currently use elongated optical paths. These systems may be large in volume and mass and not easily implemented. Further, because they use these elongated optical paths, there is not an opportunity to effectively control the optical beam in the optical system.
As set forth below, the present invention provides optical systems and methods that use a plurality of optical reflectors to fold the optical path of an optical beam used in the optical system. By folding the optical path of the optical beam, the optical system and method of the present invention can in one instance minimize the over-all volume and mass of the optical system. Further, using the folding aspects of the optical reflectors, the optical system and method of the present invention may also reduce the operating heat of the optical system. Specifically, the active reflectors of the optical system that generate heat may be spaced farther apart such that the heat from one active reflector does not add to the heating of the another active reflector to thereby reduce the operating temperature of the active reflectors. The passive reflectors used to fold the optical beam may be orientated such that the optical beam is directed to the spatially separated active reflectors. It should be noted that one or more or all of the said passive reflectors could be active.
Further, the optical system and method of the present invention may also be used to create a laser gyroscope having three optical beam paths for sensing rotation in three coordinate directions. In this embodiment, the optical system and method of the present invention positions the optical reflectors within the optical system such that the optical reflectors fold each of the optical beams into helical propagation paths. The optical paths are spaced closely together, such that the optical beams propagate within the defined volume of the optical system in a minimized volume.
The present invention also includes a method for designing an optical system to perform a desired function using an optical beam that propagates along an optical path within the optical system. The method designs the optical system such that is has at least one of a minimized size and a reduced operating temperature. The method includes the step of first determining the desired optical path of the optical beam as it propagates through the optical system to perform the desired function. Next the method determines the desired characteristic of the optical beam and the desired number of active reflectors needed to create the desired characteristics of the optical beam. Using this information, the method determines the number and position within the optical system of passive reflectors required to systematically fold the optical beam into a structured optical path within a defined volume of the optical system such that the optical beam performs the desired function of the optical system. The method of the present invention also at least minimizes the volume of the optical system or reduces the operating temperature of the active reflectors.
As an example, in one embodiment, the present invention provides a multi-pass optical amplifier and method that overcomes many of the deficiencies identified with amplifying an optical beam. In particular, the optical amplifier of the present invention is a multi-pass optical system having active reflectors that are connected to two facing construction surfaces. Located on each construction surface is a plurality of passive reflectors that fold the optical path of the optical beam into a smaller volume. The passive reflectors redirect the optical beam such that it is repeatedly directed at the active reflectors. The optical amplifier of the present invention also includes pump beam reflectors for directing an optical pump beam at the active reflectors.
In operation, the pump beam reflectors systematically direct a pump beam at the active reflectors several times to thereby energize the active reflectors. Further, the passive reflectors systematically direct an optical beam in a step-wise fashion between each of the passive reflectors and the active reflectors such that the optical beam is directed on the active reflectors several times to thereby provide an optical beam that has been amplified to a selected power level.
Importantly, the passive reflectors fold the path of the optical beam into a minimum volume, thereby minimizing the overall volume and mass of the optical amplifier. Further, the passive reflectors direct the optical beam to more than one active reflector, such that the optical beam can be further amplified. Additionally, the passive reflectors are orientated on the construction surfaces in such a manner that the passive reflectors efficiently reflect the optical beam between the passive reflectors and the active reflectors without requiring added passive reflectors and optical paths that may decrease the power level of the signal and require more space.
Additionally, in one embodiment, the active reflector comprises a laser-active layer and reflective layer connected to the construction surfaces. In this embodiment, the construction surfaces operate as heat sinks, which remove thermal heat from the active reflectors in a direction essentially parallel to the direction in which the optical beam impinges the active reflector. This, in turn, reduces distortion of the refractive index in the active reflector. Specifically, because it removes heat in a direction essentially parallel to the direction of the optical beam, the optical amplifier of the present invention does not introduce distortion in the direction perpendicular to the direction of propagation of the optical beam.
Because the optical amplifier of the present invention uses a plurality of passive reflectors to fold the optical beam into a reduced volume, the individual passive reflectors provide a way to maintain or adjust the optical beam quality. Specifically, the passive reflectors may be used to address beam divergence concerns. Further, the passive reflectors can be use to expand the beam to reduce high intensity in the optical beam. The passive reflectors could also be used to increase the diameter of the beam and thus reduce the intensity (power per unit area) of the beam. The passive reflectors could also be configured such that the active reflectors could be spaced further apart, such that the heat from one active reflector does not affect the other active reflectors. Additionally, the passive reflectors may be used to control at least one of a temporal distribution, spatial distribution, and phase properties of the incident beam.
These and other advantages are recognized by an optical amplifier according to one embodiment of the present invention for amplifying an optical beam to a selected power level while minimizing the volume and mass of the optical amplifier. The optical amplifier of this embodiment includes first and second construction surfaces oriented in facing relationship to each other. Located on each of the construction surfaces is at least one active reflector for amplifying an optical beam incident on the active reflectors and reflecting the amplified incident beam. Importantly, the optical amplifier further includes at least one passive reflector located on each of the first and second construction surfaces.
In operation, the passive reflectors fold the optical path of the optical beam and sequentially direct the incident optical beam on the active reflectors located on the construction surfaces. The incident optical beam is thus amplified to a selected power level as the incident beam is repeatedly reflected between the active reflectors. In addition to amplifying the optical beam to a selected power level, by folding the path of the incident beam into a minimum volume, the overall volume and mass of the optical amplifier is minimized.
In one embodiment of the present invention, the optical amplifier further includes at least one pump beam reflector located on each of the first and second construction surfaces. The pump beam reflectors direct an optical pump beam such that it is incident on the active reflector to thereby increase the power level of the active reflectors. In a further embodiment, the optical amplifier of the present invention includes a plurality of pump beam reflectors located on each of the first and second construction surfaces and positioned to direct an optical pump beam incident to the active reflector located on the opposed construction surface. In this embodiment, the optical amplifier further includes a pump beam reflector located on the first construction surface that is positioned such that the pump beam reflector directs the optical pump beam from the first construction surface to the second construction surface. This embodiment may also include a recursive pump beam reflector located on the second construction surface.
In this embodiment of the present invention, the optical pump beam is first reflected between all of the pump reflectors on the first construction surface and the active reflectors on the second construction surface. After which, a pump beam reflector on the first construction surface directs the optical pump beam to the pump beam reflectors on the second construction surface, where it used by the pump beam reflectors on the second construction surface to energize the active reflector on the first construction surface. After the optical pump beam has been systematically reflected between all of the pump beam reflectors and active reflector on the first construction surface, the recursive pump beam reflector redirects the optical pump beam such that the optical pump beam is systematically reflected between all of the pump beam reflectors and active reflectors in a reverse propagation path. In a further embodiment, the pump beam reflectors on the first construction surface may also include a recursive pump beam reflector that again redirects the optical pump beam signal to again follow the optical path between the pump beam reflectors and the active reflectors in a forward path.
As discussed above, the optical amplifier includes active reflectors for amplifying the optical signal. In one embodiment, the active reflector is a thin film disk having a plane for receiving the incident optical beam and a layer of laser-active material proximate to a reflective layer. In this embodiment of the present invention, the laser-active layer increases the power level of the incident beam to a first power level when the active reflector initially receives the incident beam. Further, the laser-active layer increases the power level of the incident optical beam to a second power level after the reflective layer reflects the incident beam.
In a further embodiment, the active reflector further includes a heat sink proximate to the reflective layer. The heat sink removes heat from the active reflector in a direction essentially parallel to the direction with which the incident optical beam enters the active reflector to thereby minimize distortions in the incident optical beam due to a thermally induced gradient.
As detailed above, the optical amplifier of the present invention provides a structure such that an optical signal may be amplified to a selected power level while also minimizing the volume and mass of the optical amplifier. In light of this, in one embodiment, the first and second construction surfaces approximate symmetrical confocal surfaces. In this embodiment, the optical amplifier includes two active reflectors located on each of the first and second construction surfaces and positioned such that planes of the active reflectors on which the optical beam impinges is normal to a line drawn between the axes of the construction surfaces. The optical amplifier of this embodiment further includes a first set of passive reflectors located on the first construction surface for directing the incident optical beam to the active reflectors located on the second construction surface and a second set of passive reflectors located on the second construction surface for directing an incident beam to the active reflectors located on the first construction surface.
In this embodiment, each of the passive reflectors is located on the construction surfaces such that the planes of the passive reflectors on which the optical beam are directed are parallel to a line tangent to the symmetrical confocal surface of the construction surfaces. Further, the first group of passive reflectors includes at least one passive reflector located on the first construction surface and positioned such that the passive reflector directs the incident beam toward the second group of passive reflectors, after the incident beam has been reflected between all of the first group of passive reflectors and the corresponding active reflector on the second construction surface. The optical pump beam then propagates between the pump beam reflectors on the second construction surface and the active reflectors on the first construction surface. The optical pump beams on the second construction surface may include a recursive reflector that redirects the optical pump beam in a reverse path such that optical pump beam is redirected to optical pump beam reflectors and the active reflectors. In a further embodiment, the pump beam reflectors on the first construction surface may also include a recursive pump beam reflector that again redirects the optical pump beam signal to again follow the optical path between the pump beam reflectors and the active reflectors in a forward path.
To alleviate problems with laser action between the active reflectors on the construction surfaces, the active reflectors for the second construction surface may be offset 90xc2x0 with respect to the axis of the active reflectors of the second construction surface to thereby minimize laser action between the active reflectors.
In addition, because the optical amplifier of the present invention uses a plurality of passive reflectors to fold the optical beam into a reduced volume, the individual passive reflectors provide a way to maintain or adjust the optical beam quality. Specifically, the passive reflectors may be used to address beam divergence concerns. Further, the passive reflectors can be use to expand the beam to reduce high intensity in the optical beam. The passive reflectors could also be used to increase the diameter of the beam and thus reduce the intensity (power per unit area) of the beam. The passive reflectors could also be configured such that the active reflectors could be spaced further apart such that the heat from one active reflector does not add to the heating of another active reflector to thereby reduce the operating temperature of the active reflectors. Additionally, the passive reflectors may be used to control at least one of a temporal distribution, spatial distribution, and phase properties of the incident beam.
The use of the multiple passive reflectors also provides advantages over an optical amplifier that merely uses a spherical surface to direct the optical beams. Specifically, a spherical surface will typically direct the optical beam through the sequence until it gets to the last reflector that has to redirect the beam. However, the spherical surface may not be optimal. The passive reflectors of the present invention, on the other hand, provide a desired optical beam path and provide a desired optical beam diameter. This, in turn, addresses problems with divergence. Specifically, the passive reflectors may designed such that they may control the optical beam to maintain beam quality.
In addition, the present invention also provides an optical system that is a laser gyroscope having three optical beam paths for sensing rotation in three coordinate directions. In this embodiment of the present invention, a plurality of optical reflectors are positioned within the optical system such that the optical reflectors fold each of the optical beams into helical propagation paths. As such, the optical beams propagate within the defined volume of the optical system in a minimized volume.
Further, the present invention also provides an optical system that reduces the operating temperature of the active reflectors. In this embodiment, the optical system includes both passive reflectors for folding the optical beam and active reflectors for amplifying and folding the optical beam. The active reflectors of this embodiment produce heat that may alter the characteristics of the optical system. The active reflectors are spaced a desired distance apart such that the heat from one active reflector does not add to the heating of the another active reflector to thereby reduce the operating temperature of the active reflectors. The passive reflectors are positioned within the optical system such that the optical beam is directed between the passive and active reflectors in a desired optical path to perform the defined function of the optical system.